Communication data is conventionally encrypted in order to prevent the data from being intercepted, tampered with, and so on. Ensuring a secure communication path is particularly important in wireless communication, where data can easily be intercepted.
For example, in the infrastructure mode for wireless LAN, the communication terminal and access point are provided with a standard specification known as WEP (Wired Equivalent Privacy). With WEP, an encryption key is set in the communication terminal and access point in advance, and security is ensured by using that encryption key each time communication is undertaken. However, in such a scheme, the encryption key is constantly fixed, and the strength of the encryption algorithms employed in WEP is low. For these reasons, it has been pointed out that there are many situations where WEP cannot ensure security.
To solve this problem, a standard specification known as WPA (Wi-Fi Protected Access) has been developed. WPA increases security not only by improving the strength of the encryption algorithms, but also by generating a new encryption key for each session in which a communication terminal joins a network.
In infrastructure mode, data is sent to other communication terminals via an access point, and thus the only direct communication that is performed is performed with the access point. It is therefore only necessary to ensure the security of communication with the access point. However, in ad-hoc mode, there is no access point, and thus communication is carried out directly with the partner with which one wishes to communicate. In other words, in order for terminals to carry out encrypted communication with other terminals, it is necessary for each terminal to either hold encryption keys for each of the other terminals or to utilize an encryption key that is common across the entire network.
In the case where each terminal holds an encryption key for each of the other terminals, it becomes more complicated and difficult to manage the encryption keys as the number of terminals increases.
However, utilizing an encryption key that is common across the entire network reduces the load of each terminal with respect to key management.
For example, Japanese Patent Laid-Open No. 2006-332895 discusses a method for using encryption keys in ad-hoc mode.
However, when using a common encryption key, there is a problem that it is difficult to distribute the same encryption key to new terminals that have newly joined the network.
The WPA scheme for wireless LANs uses a “group key” as an encryption key shared by multiple terminals. By implementing a four-way handshake and a group key handshake, the group key is sent from the terminal that initiated the four-way handshake to the partner terminal. However, the terminal that initiates the four-way handshake is not set when in ad-hoc mode.
Furthermore, in ad-hoc mode, there is no scheme for intensively managing the terminals that are present on a network. The terminals already joined to the network thus do not know which terminals do not hold the group key. For this reason, it is difficult for the terminals already joined to the network to discover which terminals do not hold the group key and initiate a four-way handshake.
Finally, when a terminal that has newly joined the network initiates a four-way handshake, the new terminal ends up distributing the group key, and thus the group key that has been used on the network thus far cannot be distributed to the new terminal.